why is it smaller?

[Log]

i was skimming through the internet reviewing the "bigma" (50-500)... while some people have shots at 500 saying wide open at 5.6 and some saying 6.3 am i looking at different lens reviews as the 50-500 on sigmas website says 6.3... while the only 500 5.6 they have is their prime and 300-800... did they make a new 50-500 with a smaller max. aperture?

logan

[Marc H]

I'ts quite simple.

Basically the bigma is a F4-F6.3 aperture lens, only the lens reports an aperture of F5.6 to the camera at 500MM , so the autofocus keeps working on some bodies.

Bodies like the Canon 300D 350D, 10D, 20D and 30d, to name a few, don't have autofocus on lenses with an aperture higher than 5.6. Pro bodies like the Canon 1D models go upto F8 (or something like that), before they loose autofocus.

Sigma just tricks the body, keeping the autofocus on.

[Log]

thanks for that

-Logan

[Justin Hancock]

Could that concept be explained better? Are you talking about lenses with a maximum aperture smaller than f/5.6 lose autofocus?

So say if a lense has a maximum aperture of f/3.5 but you're shooting in apeture mode at an aperture of f/22, it will still have autofocus.

Do lenses typically autofocus at their maximum aperture?


...Actually thinking out this makes sense...the lense always uses the maximum aperture (i.e. when composing the shot in the viewfinder) but only stops down to the specified aperture when the picture is taken. So it probably does autofocus at its maximum aperture.

[peripatetic]

Yes justin that's right.

They focus at max aperture then stop down for the shot.